It is known to pyrolyze organic materials to convert them to useful carbon and graphite articles, the practice dating back at least to the days of Edison. In more recent times, the development of carbon and graphite fibrous materials such as yarn, tow, cloth and felt has progressed at a rapid pace and numerous advances in their manufacture from such various organic fibrous precursors such as acrylic, cellulosic, polyvinyl alcohol, polyamide, polyimide, polyester, polybenzimidazole and the like have been made. In addition to the pyrolysis of fibrous materials, the pyrolysis of bulk resinous materials to produce a variety of glassy and amorphous carbons as well as crystalline carbons and graphites has also proceeded rapidly.
These disciplines overlap in the area of the so-called carbon-carbon and carbon-graphite composites. Carbon-carbon composites are, in general, carbon bodies reinforced with carbon fibers. The method used for the fabrication of these composites is by impregnation of low and high modulus carbon fibers in various forms such as yarn, tow, felt, or cloth in two and three dimensional weaves with a resin, pyrolytic carbon, or combinations of the two. Where a resin is used which may be a pitch or a polymer, the impregnated, cured, or uncured material is pyrolyzed by heating to temperatures sufficiently high to convert the binder to carbon. Where a pyrolytic carbon is used, a fibrous skeleton is impregnated with carbon by the thermal decomposition of a hydrocarbon gas such as methane. Modifications of these processes include reimpregnation of resins into carbonized bodies to increase density and properties, or impregnation of pyrolyzed fiber-carbonized resin composites with pyrolytic carbon to effect the same improvements. During pyrolysis, the matrix shrinks resulting in the development of stresses and/or cracks in the binder and a possible weakening of the bond between the fiber and the matrix. The impregnation of a carbon skeleton with pyrolytic carbon (produced by the thermal decomposition of a hydrocarbon gas) leads to a similar condition which is due to the anisotropic shrinkage of the matrix on cool down from process temperatures (1100.degree. C. to 1300.degree. C.). The above dimensional changes are responsible for the relatively low shear strengths of these materials. With increasing heat treatment temperature and high temperature (2000.degree. C. to 3000.degree. C.) heat treating cycles the dimensional changes in the binder or matrix phase increase leading to further reduction in the shear strength of the material.
The use of nondirectional fiber geometries such as in felted or 3-D weave patterns tends to obscure the above condition to some extent since the purpose of such fiber geometries is to reinforce the material in all directions. However, this scheme does not eliminate the basic problem of dimensional changes in the binder during processing.
A substantial advance could be made in the art of carbon-carbon composites if the problem of differential fiber-matrix dimensional changes could be eliminated. One approach that has been tried with limited success is to employ precursor fibers or partially carbonized fibers in a resin matrix instead of fully carbonized fibers in an attempt to match the shrinkage of the fibers to the matrix during pyrolysis.
This invention takes a markedly different approach to solving problems of fiber-matrix interactions, namely the elimination of the matrix entirely, and relies instead upon the plasticity of suitable precursor fibers to effect fiber bonding by a compression molding step prior to pyrolysis. In this way the strength and stiffness of carbon fibers can be exploited to produce articles having properties tailored to specific end uses, without introducing weaknesses associated with fiber-matrix interaction.